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Male Reproductive Senescence

A late 1800s image of a gorilla male, albeit one probably not thinking about reproductive senescence

How do you check the cardiovascular function of a large gorilla male? Not very easily, as this story chronicled: usatoday.com/how-does-zoo-test-an-apes-heart. But it’s an important effort, given that one of the major health challenges facing aging captive gorilla males is heart disease. While the recognition of chronic diseases in captive apes is notable in itself, I wish to push in a different direction—that of male aging and reproduction.

I wonder if a vet working with aging male gorillas has thought about whether the gorillas are healthy enough for sexual activity. I can almost hear the echo of some erectile dysfunction TV advertisement—do not take Viagra if you are on nitrates…consult your doctor. Do zoo biologists (and gorillas) need to consider whether older gorilla males can have sex, without a concern over a heart failure? In a related vein, could the offspring of older gorilla male fathers be more vulnerable to schizophrenia, the subject of a recent human study and ensuing media flurry? nytimes.com/fathers-assessing-the-risks.

When it comes to aging, great apes and humans are not playing on a level field. Gorillas appear to have a faster life history than other great apes and humans. Gorillas reach puberty at earlier ages and appear to senescence (lose function with age) at younger ages than other great apes. Orangutans tend to have older ages of puberty, perhaps of senescence too, when contrasted with gorillas, chimpanzees and bonobos. Human hunter-gatherers reach puberty at later ages and senescence at later ages than all of those great ape species. Human fathers reproduce at older ages than chimpanzees (for which the best great ape data are available: humans-have-older-fathers-chimpanzees), though that is likely true compared with the other great apes, including any older potential gorilla males with concerns over heart health.

Human males nonetheless face reproductive senescence. We’re not designed to live forever, and our reproductive equipment is not designed to function indefinitely. Instead, evolutionary influences have shaped age-related decline in various measures underlying the potential to father offspring, and over offspring quality, even if those ages in our species are delayed in comparison with our closest ape kin.

Males are less likely to get in physical fights with advancing age. Oh, that’s a good thing, right? The male motivation to respond with physical aggression to status sleights changes with advancing age, arguably in part because it is optimized to function highest among young adults who are learning the ropes, with the costs of failure at fights potentially increasing at advancing ages as males have more (like caring for their kids) to lose. Male sexual desire is contingent upon a variety of factors, and two of those are age and health status. As men age, their libido may decrease, arguably too because the reproductive payoffs to high libido favor it being highest in early adulthood and declining with later age.

Major aspects of human male senescence make sense when viewed within a light of declining selection on maintaining them for physical aggression or sex with advancing age. Males lose muscle and bone, but there’s also less pressure to use that muscle in fights. Male testosterone levels tend to decline with advancing age, though that is also subject to population variation (i.e., populations in which younger male testosterone levels never get that high also have less far to drop with advancing age). Other aspects of the male hypothalamus-pituitary-gonadal (HPG) axis change with age, including a rise in luteinizing hormone levels, indicative of the axis responding less sensitively to hormone feedback with age. Older males are more vulnerable to erectile dysfunction, suggesting that even if a mating opportunity is available, the chances for conception may diminish. Male sperm quality and ejaculate volume decrease with age. Those reports of the deleterious mental health effects that may be transmitted by older fathers to offspring speak further to male reproductive senescence. There is evidence too of male age-related decreases in vitro fertilization (IVF) success.

None of this happens overnight. The arc of a man’s reproductive life looks pretty gradual when seen from afar. Most of these age-related changes become more measurable at quite later ages, such as erectile dysfunction in men in their 60s and later. The changes may still allow some reproductive surprises, such as a record-setting 96 year old man in India having a child just a few years ago (abcnews./farmer-96-claims-to-be-worlds-oldest-new-dad). There is population variation in male age-specific fertility, depending upon the marital system and other factors, meaning that the ages by which men become fathers depend on context. Yet for the vast majority of men, whether among foragers or in the U.S. today, the ages at which men father children still tend to be concentrated in early and middle adulthood rather than in late life. It’s a marvelous endeavor trying to discern the ultimate and proximate dimensions of male reproductive senescence. Our hearts (and minds, and reproductive equipment) seem to care.

References

Bribiescas, R. B. (2010). An evolutionary and life history perspective on human male reproductive senescence. Annals of the New York Academy of Sciences, 1204, 54-64.

Not to disagree with anything in the article, but I would point out the obvious -- for most long term married couples, the first thing that stops a man from having children later in life is his wife's menopause, not the husband's gradual sexual decline.

Today it's not uncommon to find a healthy man in his 60's who is still able and willing to have frequent sex. But there is no chance that his wife of the same age can bear his children.

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